Many industrial processes involve a moving sheet of material to which a coating is applied, a liquid absorbed, a material laminated, or the physical characteristics of which are changed. It is often only possible to measure and thereby control this change in the material differentially; i.e., by measuring the moving sheet both before and after the change has occurred. In such a differential measuring scheme it is important to make both the before (Base) and after (Gross) measurements at the same spot unless the material is sufficiently uniform so that the measurement errors due to the lack of same-spot measurement are acceptably small.
For a moving sheet process, the velocity of the sheet is often itself a variable. Therefore, to maximize the amount of usable material formed, it is desirable to be able to immediately make same-spot measurements independent of sheet velocity or acceleration.
One method of measurement currently in use, long-term averaging, uses a differential measuring scheme with no attempt at same-spot measurement. To overcome the errors due to a lack of same-spot measurements the readings are averaged using various averaging techniques to achieve an acceptable measurement. In applications where the gross to base ratio is low, or where the base is non-uniform, the measurement results obtained with this technique are particularly poor.
A second method; i.e., same-spot differential measurement for a constant line speed, gives accurate results if the line speed remains constant, but errors are introduced whenever the line speed changes. To prevent such errors from becoming excessive during a line speed change, the traversal of the sensors is terminated and restarted at the new line speed. This action results in sections of the sheet not being measured during a line speed change.
A third method involves monitoring changes in line speed and line acceleration to set a new scanner traverse rate which then remains constant irrespective of any further line speed changes during the scan. Such discontinuous methods do not account for inacurracies that arise when the line speed changes during a scan interval, resulting in loss of measurement accuracy until the scanning rate is eventually updated.